This invention relates to a rocker arm pivotally mounted between a cam and a valve in a valve-moving mechanism for driving intake and exhaust valves in an internal combustion engine, and particularly a rocker arm having a roller at its portion abutting the cam.
This type of rocker arm generally has its roller rotatably supported on one end of the rocker arm body by a pin, and between the outer peripheral surface of the pin and the inner peripheral surface of the ring, a roller housing space is formed to house a plurality of rollers in a full type form.
Since the ring portion of this rocker arm is used under severe lubricating conditions, in order to improve durability and wear resistance of the rocker arm, as described in Japanese patent publications 10-47334, 8-93418 and 8-109959, improvements in the roller material have been mainly pursued, such as modification of steel material and use of a ceramic material.
An object of this invention is to prolong the life of a rocker arm in which a plurality of rollers are housed in a full type form between the ring and the pin from a viewpoint completely different from the idea of improving the material.
The inventors of this invention have found out that as a factor having an influence on the life of a rocker arm in which a plurality of rollers are housed between a ring and a pin in a full type form, clearances in the circumferential and radial directions provided between the outer peripheral surface of the pin and the inner peripheral surface of the ring play an important role, and they analyzed the relation between these clearances and the life.
In this invention, the clearance in the radial direction refers to a difference between the inscribed circle of the rollers and the outer periphery of the pin with the rollers in contact with the inner peripheral surface of the ring. The load rate of the rollers depends on the size of the clearance in the radial direction.
As the clearance in the radial direction increases, the load rate decreases, so that the surface pressure per roller increases. On the other hand, as the clearance decreases, the load rate increases, so that even though the surface pressure per roller decreases, due to the influence of the shape of the inner peripheral surface of the ring (roundness and cylindricity), the clearance can partially become negative. This can cause seizure.
In particular, since the ring has a thick wall thickness compared with an outer ring such as shells and has high rigidly, under severe lubricating conditions, it is necessary to avoid premature seizure due to clearances being too small. Also, difficulty in assembly results from clearances being too small.
Also in this invention, the clearance in the circumferential direction refers to a clearance per roller which is determined by dividing by the total number of the rollers the clearance formed between the first and last rollers when all the rollers have been displaced together into contact with one another and with the rollers abutting the inner peripheral surface of the ring.
As this clearance in the circumferential direction increases, skew of the rollers increases, so that an excessive load due to an edge load is produced. This shortens the life. On the other hand, as the clearance in the circumferential direction decreases, there occurs a problem of wear due to a struggle between the rollers and heat buildup due to slip.
The clearances in the radial and circumferential directions were heretofore set experimentally at 0.009-0.22 mm in the radial direction and 0.003-0.011 mm in the circumferential direction, respectively, in view of ease of assembly of the rollers. But according to this invention, the clearance in the radial direction has been changed from the conventional 0.009-0.022 mm to 0.005-0.018 mm to extend the life in view of the relation between the clearance in the circumferential direction in this range and the life. It was found out that it is possible to extend the life by setting the clearance in the circumferential direction at 0.012-0.018 mm, which is larger than the conventional range of 0.003-0.011 mm.
Other features and objects of the present invention will become apparent from the following description made with reference to the accompanying drawings, in which: